Rays and skates are in the superorder of cartilaginous fish, known as Batoidea, where there are approximately 560 species in thirteen families. They are closely related to the Elasmobranchii family, of sharks. Rays have a flattened body, enlarged wing-like pectoral fins that fuse to the head and gill slits that are placed on their ventral surfaces. Like sharks, they are cartilaginous marine fish, which means that they have a boneless skeleton made of a tough, elastic substance. Most species of rays live on the sea floor, in a variety of geographical regions – many in coastal waters, few live in deep waters to at least 3,000 meters (9,800 feet). Most are distributed in tropical and subtropical marine environment, only a few species, like manta rays, live in the open sea and very few live in freshwater. Some rays can live in brackish bays and estuaries. The bottom-dwelling rays breath by taking water in through the spiracles, rather than through the mouth as most fish do, and passing it outward through the gills.